A solid-state laser typically comprises a selectively doped glass rod or a material such as Nd:YAG ruby and the like rods. The laser rod is positioned within a suitable pump enclosure including a pair of mirrors or resonators which may be either positioned on the ends of the rod itself or external to it. Generally, the resonators are two parallel flat reflecting surfaces, one of which is totally reflecting at the desired wavelength and the other is partially transmitting at the desired wavelength. However, a number of improvements have been disclosed wherein the laser cavity employs at least one concave or spherical reflector. See, for example, U.S. Pat. Nos. 3,137,827 and 3,253,226. In the latter patent, a pair of spherical resonators are spaced apart a distance differing from their confocal distance to increase the effective path length to several times that of the actual spacing. In U.S. Pat. No. 3,055,257, a confocal cavity is provided to obtain a higher energy density through concentration of the light energy by the cavity than is generally provided by flat parallel resonators. The higher energy density, in turn, reduces the requirements on the pump source. In U.S. Pat. No. 3,365,678, a self Q-switching laser is disclosed utilizing photochromic glass and converging lenses to concentrate radiation upon a localized area of the glass to increase the efficiency thereof.
Generally, a pumping light or flashlamp is employed in solid-state laser systems to raise the atoms in the laser material from the ground state to an excited state. A Q-switched laser uses a rapidly opening optical shutter, such as an electro-optically active material in combination with a polarizer, to suppress laser action until a large population inversion of excited state atoms or ions in the laser rod has been achieved. Some of the atoms return to the ground state spontaneously emitting photons. With the optical shutter closed, the laser losses exceed the laser gain and no amplification of the spontaneous emission occurs. When the shutter is opened, however, the laser gain exceeds the laser losses and a beam of coherent light flashes through the end of the laser rod which is only partially reflecting. The laser light exits as an intense, highly directional, monochromatic beam.
Certain materials, particularly glasses co-doped with neodymium and a bleachable ion such as uranium dioxide, act as self Q-switching or regularly spiking lasers. The pump lamp excites the laser ions, producing an inverted population, and the conditions normally necessary for laser action to occur. At the same time, some of the flashlamp radiation excites the bleachable filter ions, producing an induced absorption which inhibits the lasing action from normally occurring. As pumping continues, the population inversion of the laser ions continues to build up, with a consequent increase in the intensity of the spontaneous emission from these ions. This, in turn, serves to bleach the induced absorption, partially counteracting the effect of the flashlamp. In time, the induced absorption is reduced sufficiently to permit a laser pulse to appear. This pulse is high in amplitude, and very narrow, and resembles a pulse produced by external Q-switching. The pulse partially depletes the population of the excited laser ions, and when this population has fallen below some critical level, laser oscillation stops. During and after this period, the flashlamp continues to pump, eventually restoring both the opacity of the bleachable ion and the population inversion in the laser ion necessary to produce a second pulse. The process than repeats itself, and continues to do so until the flashlamp characteristics fall below some critical conditions.
There is a strong tendency, however, for self Q-switched laser materials to lase over only a small portion of the entire rod volume. Laser rods are fabricated as small as 0.08 inch diameter by 1 inch in length and possibly as large as several inches diameter by 72 inches long; a commonly used rod size is one quarter of an inch in diameter by 3 inches long. When the laser rod is operated utilizing flat resonators, it has been observed that as the rod self Q-switches and releases its pulses of energy, only a small fraction of the rod volume is contributing to an individual energy burst. For example, a small filament in the rod approximately only a millimeter in diameter is actually lasing.
The Q-switched burst of energy occurs as soon as a portion of a Q-switched laser rod becomes transparent between the resonators. Thus, one of the disadvantages of prior art self Q-switched lasers is that only a small filament contributes to the lased energy. Also, self Q-switched lasers often produce a number of randomly spaced spikes of energy.
Accordingly, it is an object of the present invention to overcome the inherent disadvantages of prior art self Q-switched laser by providing a laser rod having a larger effective volume for storing more energy which can be released in a single burst. It is a further object of the invention to obtain a greater energy output for the same energy input by making the entire laser rod function as a unit rather than as individual filaments.